From Wikipedia: Historian Nicholas Rogers, exploring the origins of Halloween, notes that while "some folklorists have detected its origins in the Roman feast of Pomona, the goddess of fruits and seeds, or in the festival of the dead called Parentalia, it is more typically linked to the celtic festival of Samhain or Samuin (pronounced sow-an or sow-in)", which is derived from Old Irish and means roughly "summer's end". A similar festival was held by the ancient Britons and is known as Calan Gaeaf (pronounced kalan-geyf).
The festival of Samhain celebrates the end of the "lighter half" of the year and beginning of the "darker half", and is sometimes regarded as the "Celtic New Year". The celebration has some elements of a festival of the dead. The ancient Celts believed that the border between this world and the Otherworld became thin on Samhain, allowing spirits (both harmless and harmful) to pass through. The family's ancestors were honoured and invited home whilst harmful spirits were warded off. It is believed that the need to ward off harmful spirits led to the wearing of costumes and masks. Their purpose was to disguise oneself as a harmful spirit and thus avoid harm. In Scotland the spirits were impersonated by young men dressed in white with masked, veiled or blackened faces. Samhain was also a time to take stock of food supplies and slaughter livestock for winter stores. Bonfires played a large part in the festivities. All other fires were doused and each home lit their hearth from the bonfire. The bones of slaughtered livestock were cast into its flames. Sometimes two bonfires would be built side-by-side, and people and their livestock would walk between them as a cleansing ritual.
Another common practise was divination, which often involved the use of food and drink.
The name 'Halloween' and many of its present-day traditions derive from the Old English era.